The invention relates to a method and arrangement for inspecting a weld seam between sheet metal blanks with projected light.
The welding together of sheet-metal blanks to form larger components (so-called tailored blanks), which are subsequently subjected to a forming process, is known. One example of a field in which such tailored blanks are used is the motor vehicle industry. The welding together of the blanks may be performed by mash seam welding or by laser welding. Because the weld seam is subjected to the forming process together with the blanks, it is essential that the entire seam should be of sound quality. The blanks to be welded are fed to the welding station with their joint edges juxtaposed. It is important that the focused laser beam should follow the joint edges of the juxtaposed blanks as accurately as possible. To achieve this, use is made of devices which determine the precise track of the edges ahead of the welding zone, thus enabling the laser beam to be guided accordingly during welding. One known device does this by projecting a line pattern of constant intensity transversely across the edges; this pattern is imaged by a camera, and the image obtained is analysed by computer in order to derive from the trace of at least one of the lines the track of the edges, or as the case may be of the gap, between the blanks at that point. In this way, the edges and the gap are tracked along their entire length ahead of the welding zone and the laser beam is controlled accordingly as welding is performed.
It is also known to employ basically the same method for optically inspecting the weld seam after welding. In this case, one or more lines of light are similarly projected across the weld seam (at right angles to the seam or obliquely), and a camera captures an image of the trace of the projected lines from above. The image obtained is analysed by computer using known image processing methods in order to detect any defects in the seam which disturb the trace of the lines. Depending on the configuration of the seam, differences in thickness between the blanks and their reflectivity, and the nature of any weld defect which may be present, it is possible that the optical weld seam inspection which has been described does not clearly recognise a defect which is present, or that relatively high degree of uncertainty prevails in the analysis as to the existence of a defect.
Therefore the object of the invention is basically to provide an optical inspection of a weld seam which allows better and more certain detection of any weld defects which may occur.